Western Hwy Duplication Project – Flood Investigation
TGM was appointed to extend the earlier Bonacci Water investigation into flood investigation along the alignment of the VicRoads Western Highway duplication project which is providing a second carriageway for the Western Highway between Beaufort and Stawell in central Victoria. Works included responding to review by the Wimmera Catchment Authority (GHCMA) and to amend the earlier report. This investigation was conducted in three phases as follows;
- Collection and forensic investigation of existing data and information to create a reliable digital terrain model (DTM), definition of hydrology of the region and understanding of recent flood events;
- Development of preliminary hydrology and hydraulic models that were used to test the accuracy of the DTM. The accuracy of the preliminary results was confirmed using existing knowledge; and
- Finalise the hydrology and hydraulic models using calibration and verification techniques. In addition, the results from the hydraulic models were used to refine the DTM for the region.
The study produced a range of information about flooding generated by design storm events (1 year to 100 year ARI) as specified in Australian Rainfall and Runoff published by Engineers Australia and from the January 2011 storm events. This investigation included considerable detail and provides information about flooding adjacent to the Western Highway and within the many waterways that interact with the highway alignment. A wide range of additional information was also provided including stormwater peak discharges from catchments and the depth of flood waters crossing the road.
- This investigation combined data from photogrammetry (10 m contours) provided by DSE, LIDAR (1 m contours) from VicRoads, field inspections and feature survey to define stormwater catchments impacting on the Western Highway and to create a digital terrain model (DTM) of the area surrounding the existing highway.
- Mapping derived from photogrammetry and LIDAR can be unreliable for analysis of flooding in areas with vegetation, steep gullies and flat terrain. There is always a need to conduct carefully selected field inspections and feature surveys as a “ground truth” of topography derived from remote sensing. This project included this important process to remove any anomalies from the raw data in the refinement of the digital terrain model (DTM).
- A suite of calibrated and verified terrain, hydrology and hydraulic models was created for the existing Western Highway between Stawell and Beaufort.
- The hydrology and hydraulics of the region was calibrated using continuous rainfall (6 minutes intervals) from Ararat and streamflow from the Upper Hopkins River.
- The magnitude of peak discharges was verified using multiple methods including flood frequency analysis at the Upper Hopkins stream gauge, Rational Method calculations and use of design storm events and temporal patterns in a calibrated hydrology model. This provided confidence in the likely peak discharges generated by 100 year ARI storm events.
- The models were verified using information from historical photographs, historical flood marks from the Great Western area and feedback from stakeholders.
- The hydrology and hydraulic models created during this investigation were able to reproduce the extent and depth of flooding from the storm events in January 2011.
Western Highway Extension Section 1B (Burrumbeet to Beaufort) – Stormwater Harvesting
Hydrologic analysis of entire construction site for the purpose of estimating additional runoff volume available for storm water harvesting for construction purposes. Included liaison with Southern Rural Water and assistance in application for stormwater extraction license.
Western Highway Extension Section 1B (Burrumbeet to Beaufort) – Mt Emu Creek
High level advice and assessment and consideration of options to reduce afflux at proposed bridge during tender phase of design and construct tender, resulting in upgrade to existing upstream bridge alternative to oversized bridge and significant savings to the client. Completed as part of TGM’s role as drainage designer on the bid team.